[Diy_efi] Throttle before turbo on EFI

[Mike]

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This was discussed tangentially on the thread about using the TB to
control boost,

Biggest problem (if you also get rid of TB after turbo) is the comp
seals face partial vacuum for a large period of time in normal cruise,
not generally designed for it (AFAIK)

rgds

mike



At 11:03 PM 4/15/03 -0700, you wrote:
>How's this for a idea? I'm not talking about your normal carbed
"draw-through"
>type of setup. Rather a intercooled large turbo (aprox T66) with a huge
(105mm)
>throttle body in front of it.
>
>Let's not let the things associated with older draw-through applications
be the
>basis of critisim of the above idea. I know very well those problems.
>Specifically they are vacum at the compressor inlet (due to insuffciently
sized
>carbs causing a pressure drop), and having to flow fuel through a compressor.
>Those concerns won't exist in my case. With a 105mm throttle body, it's
just as
>big as a 4" inlet to the turbo, which is what I have. So at WOT, there
should be
>full atmospheric pressure at the compressor inlet. I'm still EFI, and use
port
>fuel injection, so no fuel will pass through the turbo or intercooler.
>
>Advantages I can think of.
>1. No blow off valve needed
>2. Quicker return of boost between shifts (compressor doesn't slow down
nearly
>as much or decelerate suddenly)
>3. Longer turbo life? I'm guessing here but the removal of harsh
transients to
>the compressor would put less stress on the shaft, bearings, etc.
>4. Less pumping losses at cruise/idle? The turbine will no longer be
pushing on
>a compressor that is trying to push air that has no where to go (against a
>closed or barely open throttle).
>5. Quicker spool up? Won't the compressor be at a higher RPM in general
>(cruise), making for quicker spool once the throttle is cracked?
>
>Disadvantages. All of these are a big MAYBE. They may not be a concern at
all.
>1. Greater tendency for oil seal failure in turbo. Off throttle vacumn
will be
>sucking on the bearing. However if you think about it, the strain on the
seal is
>just the delta pressure between the oil pressure and the compressor pressure.
>Having 15" of vacumn in the compressor is the same as extra 7.5psi or so
of oil
>pressure. So it shouldn't make a difference.
>2. The generally higher compressor RPM will cause more wear. I'm not sure if
>this is even an issue, or if the savings in transient wear will more than
make
>up for it.
>3. Time to decel. The extra volume in the system may cause the engine to
still
>make power for a brief instant after the throttle is lifted, as the engine
>gobbles up the air that in a regular system would shoot out the blow off
valve.
>In my case, I don't need to worry about this. My engine computer shuts off
>fueling when the throttle is lifted, and above 1500RPM




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